The invention relates to a method of printing and to an apparatus for use in that method and, more particularly, the invention relates to the printing on an image directly from a printing plate onto a surface.
Presently there are four most commonly employed printing methods. In one, lithography, a flat-surfaced lithographic plate, which has image portions that hold ink and repel water and non-image portions that hold water and repel ink, transfers the ink to a blanket, e.g. a rubber-covered cylinder, and the blanket transfers the ink to a surface on which the image is to be printed. Hereinafter such a surface will be referred to as paper, although it will be appreciated that the image may be printed on other surfaces, as is well known in the art, and the image on the plate to be printed will be referred to as the printing image. Letterpress is another printing method in which the ink on raised surfaces of a metal plate defining the printing image thereon is transferred to the paper. In gravure printing, the ink is applied to the paper from hollows that are etched in the plate surface, and in screen-process printing ink is forced by a squeegee or the like through open areas in a screen onto the paper. The equipment for producing printed copy, especially at relatively high speeds, is relatively complex, bulky, expensive, and complicated to use to produce printed images of high fidelity, i.e. correspondence with the original copy.
Line copy is that type of printed image that ordinarily has no intermediate tones, and such line copy is produced by depositing a uniform amount of ink onto the paper by the printing surface wherever the latter bears a printing image; where there is no printing image, the paper remains clear so that the resulting line copy printed image is formed by the contrast between the clear or unprinted paper surface and the lines, type matter, dots, etc. printed thereon. To enhance tonal gradations from white to black or even from one color to another a halftone screen, for example, is merged with the original image on the original copy or with the printing image on the printing surface. The printed halftone image on the paper is formed of a plurality of black dots and white areas, where there is no image, as is well known. Moreover, copy with intermediate tones of grey is known as continuous-tone copy, and an image printed from copy will be similar to a halftone image but with same dots of various sizes effecting the more continuous tonal gradation. Halftone and continuous-tone printing usually produces printed images that have a grainy effect with boundaries that often do not correspond exactly with those of the original image and, accordingly, do not have a high degree of fidelity.
Color copy is printed using plural printing surfaces associated with respective dual printing cylinders, impression cylinders, and the like with complex inking and roller mechanisms to deliver ink of different respective colors. The printing of colors thus further increases the size of the printing equipment, and, for example, in the common method of magazine production, which employs such complex color printing equipment, massive ink dryers also are required. Also, since the different colors are printed at different locations a loss of fidelity may too easily occur. The distance between printing units affects reproduction fidelity by the constant changes occurring by the pressroom atmosphere and conditions affecting the quality of the ink formula and the exposure through a plurality of inking rollers from the ink reservoir to the printing plate surface.
A color mixing effect may be obtained when printing halftone and continuous-tone color copy by printing some of the localized dots in one color and some in another with the eye then mixing those dots, for example, combining a plurality of discrete blue and yellow dots that would appear visually as green. A disadvantage to printing color halftone and continuous-tone images is that the plurality of color dots are not all located exactly at the various boundaries of the original image and, therefore, fidelity is reduced. Moreover, in the color halftone printing process, a separate plurality of inking rollers and halftone printing surface screens are required at spaced-apart locations, of course, with each step away from the original copy there is a corresponding loss of fidelity.
Thus, the prior art printing methods require relatively large and complex equipment including presses with multiple printing units and compound inking systems, and a number of separate pieces of equipment are required to produce a negatives, halftones and plates as interim steps in the printing process from the original copy to the printed copy. Those printing methods are relatively expensive for both equipment and labor and are limited in the degree of true reproduction capability or fidelity.
As used herein, shade indicates a difference in color whereas tone means a difference in the lightness or darkness of a given color or of black to white, e.g. tones of gray.